The hackerrank test questions are usually simple programmer (algorithmic, data handling) questions, usually you will need to write a simple program, a few classes, invoke them, read data in, write result out. No multithreading no specific technical knowledge needed, just basic IO and language.
The trick and the reason why You can fail easily : they expect 100% ready and working pieces of software, if You miss a line which is trivial like reading in or writing out a value from stdin , then you failed.
Even if the task is simple, the time for exam is VERY SHORT (90 min for two tasks), you will NOT have much time for debugging, and if your software not working (even if 90% you typed in) then again You get zero points.
